It is known that the use of physico-chemical processes is common to most treatments applied to various types of water and that these treatments essentially consist of:
clarification of surface water for consumption or for industry; PA1 clarification of municipal sewage, storm water or industrial waste water; PA1 decarbonization; PA1 removal of phosphates; PA1 etc. PA1 coagulation: a step of neutralization of the colloids using a metal salt, generally a trivalent iron or aluminum compound, in order to form a microflock. This coagulation step may be carried out in one or more steps; PA1 flocculation: a step of agglomeration and growth of the microflock. This agglomeration step takes place by virtue of the addition of a polyelectrolyte (or polymer) downstream of the coagulation step; PA1 settling: a step of separation of the flock from the interstitial water, causing the formation of sludge on the one hand, and-of clarified water on the other hand. PA1 flocculation with a contacting mass, which has allowed the quality of the flocks to be improved, the volume of the reactors to be reduced and the clarification to be improved. This is because the microflocks of the coagulation have a greater chance of agglomerating and of growing as the reaction medium contains a high density of particles: the rate of flock formation is proportional to the number of free particles in the suspension; PA1 lamellar settling, carried out by introducing inclined plates or tubes in the settling tanks. This technology has made it possible to reduce the size of the settling tanks by from 50 to 70%. PA1 1. recirculated presettled sludge: an example of this technique is described in FR-A-2,553,082; PA1 2. fine ballasts, such as microsand: an example of the use of this technique is described in FR-P-1,411,792 and in FR-A-2,627,704. PA1 1--the contacting mass is generated by the process, and is therefore available without any quantity limitation, depending on the requirements of the process; PA1 2--in the flocculation reactor B, the mass of sludge due to the recirculation is very high compared to the suspended matter brought in by the raw water. As a result, the system is insensitive both to significant increases and significant decreases in the amount of suspended matter in the raw water; PA1 3--the contacting mass presents a very high specific surface area or spatial occupation because of its expanded structure and its low relative density; by way of example, 1 gram of flocculated sludge in one liter (average concentration in the reactor) occupies, after settling for approximately 5 minutes, a volume equal to 100 ml. This very high specific surface area or spatial occupation considerably increases the probability of contact between the flocks and the very fine particles, coagulated colloids and micro-organisms, and therefore the probability of "trapping" this suspended matter very efficiently; PA1 4--because of the continuous recirculation of ever-reflocculated sludge, the latter densifies. Thus, the extracted sludge is highly concentrated (two to ten times more concentrated than the sludge in most apparatuses); PA1 5--this technique makes it possible to achieve relatively high treatment rates. Thus, when clarifying river water, the announced rates through the lamellar modules of the settling tank are between 10 and 25 m.sup.3 /m.sup.2.h, which corresponds to settling velocities UD (the flow rate/raft area ratio of the settling tank) of 6 to 15 m/h. These velocities are in fact limited by the limiting mass flux (Fml) of the flocculated suspension expressed in kg of suspended matter flowing per m.sup.2 of settling tank raft and per hour (kg/m.sup.2 /h). PA1 6--often such apparatus are followed by a filtration system. This is the case with river-water clarification. The filtration is characterized by the quality of the filtered water and by the filtration cycle time (the period of operation after which the maximum caking of the filter is achieved, requiring the latter to be washed). In the case of settling tanks with sludge recirculation, the filtration cycle time is generally greater than 24 h. PA1 7--increasing the settling velocities UD is possible, but at the cost of increasing the dose of polyelectrolyte. However, an excess of polyelectrolyte reduces the filterability of clarified water (increase in the time of the test), which means, on a filter, surface clogging and therefore a reduction in the filtration cycle time. PA1 accelerated flocculation, by virtue of the use of a contacting mass having a high specific surface area (or spatial occupation); PA1 increase in the settling velocities, resulting from the addition of ballast to the flock. PA1 for equivalent contacting mass (by weight), the ballast offers a contacting surface area or percentage of spatial occupation which is much less than the sludge. By way of example: PA1 the system is sensitive to sudden pollution caused by lack of availability of binding sites on the ballast (the contacting mass is limited to a maximum of 5-10 g/l); PA1 the system has a lower performance with regard to so-called "sensitive" pollutants (helminth eggs, micro-organisms, micro-particles, traces of complex organic compounds, pesticides etc.); PA1 the low concentration of extracted sludge, resulting from the need to clean the ballast as fully as possible--this concentration is at least 10 times lower than that measured on apparatus using a sludge contacting mass--and frequently involving the installation of a complementary unit, downstream of the settling tank, for thickening the extracted sludge.
These types of physico-chemical treatments always comprise the following successive steps:
Over the last thirty years or so, the state of the art relating to such a physico-chemical treatment has evolved considerably as a result of the appearance of two technologies:
The current technological trend is towards improving the flocculation conditions, which are key in determining the quality of the treated water and in obtaining high settling velocities.
At the present time, modern settling tanks use two types of contacting masses in the flocculation reactor:
The present invention relates to improvements made to apparatus employing recirculated presettled sludge, these improvements being such that the apparatus, while still maintaining their specificities and their advantages, operate at much greater settling velocities.
Before describing the improvements, the advantages and disadvantages of this known technique, using a sludge contacting mas, will be explained.
FIG. 1 of the appended drawings shows diagrammatically a physico-chemical treatment plant employing this technique. This figure shows diagrammatically, at A, the coagulation reactor, at B, the flocculator and, at C, the settling tank. These are plants well known to those skilled in the art and, under these conditions, they will not be described.
Thus, as may be seen in this FIG. 1, the contacting mass in the flocculation reactor B consists of the recirculation of part of the sludge which has settled in C, this recycled part of the sludge being introduced into the flocculator B via the pipe 16 and the recirculating pump 18. The recirculated sludge volume represents between 0.5 and 4% of the treated volume. The recirculated sludge, the raw water and the polyelectrolyte are brought into contact with each other in a highly turbulent zone, the polymer, as may be seen in FIG. 1, generally being injected near the propeller 10 of the flocculator B. The excess, concentrated sludge is extracted and removed.
The advantages of this flocculation technique using recirculated presettled sludge as the contacting mass are the following:
The mass flux is the limiting factor which determines the limiting theoretical settling value Udl. This value is also related to the concentration CR of the sludge in the reactor B, expressed in kg/m.sup.3 : EQU Fml=CR.times.Udl
i.e.: Udl=Fml/CR.
If the settling velocity UD applied to the settling tank C is equal to or greater than Udl, there is clogging. The lamellar settling tank is effective as a finisher, but it is incapable of retaining a sludge bed.
It is therefore necessary to check that the mass flux applied to the settling tank is less than Fml or that the settling velocity UD applied to the settling tank is less than Udl.
For example, in the case of river-water clarification, the limiting mass flux is generally less than or about 20 kg/m.sup.3.h. The concentration CR required for good flocculation is approximately 1 kg/m.sup.3. The limiting settling velocity Udl is then 20m/h, hence the velocity UD applied to the settling tank is less than 15 m/h for safety reasons;
In order to determine the ability of the clarified water to be filtered using correct filtration times, tests representative of the clarified-water filter-ability are carried out. Thus, one of the tests that can be used consists in measuring the time necessary to filter 250 cm.sup.3 of clarified water on a 5 .mu.m membrane under a vacuum of 8.times.10.sup.4 Pa. The water will be more easily filtered in a shorter time. In the case of a settling tank operating with a settling velocity UD equal to 15 m/h, the filterability is approximately 30 seconds;
The present invention aims to improve the technique explained above, for the purpose of increasing the settling velocities thereof, without affecting the filterability of the clarified water.
Consequently, the subject of the present invention is a process for the physico-chemical treatment of effluent, especially of surface water intended for consumption, The process includes the successive steps of coagulation, flocculation and settling. A contacting mass is introduced into the water coming from the coagulation step, and therefore during the flocculation step. This contacting mass consists of part of the densified sludge resulting from the settling step and recycled in the flocculation step. This process is characterized in that at least part of the polyelectrolyte ensuring flocculation is injected into the sludge-recirculation circuit.
It has been found that operating in this way has the unexpected and advantageous effect of increasing the limiting mass flux, the settling velocity, the filterability of the raw water and the concentration of the extracted sludge.
The process, forming the subject of this invention and such as defined above, may be implemented in a flocculation step using a ballast as the contacting mass.
It is known that, according to this technique, the contacting mass is obtained by adding, upstream of a flocculator, a fresh or recycled ballast after cleaning. The means making it possible to separate and regenerate the ballast which is to be recycled in the flocculator are means well known to those skilled in the art and, under these conditions, they will not be described.
When implementing this technique, the ballast generally consists of sand and the continuously extracted materials amount to approximately 5% of the volume of water treated by the settling tank; these extracted materials, laden with sludge coating the microsand, must be treated so as to regenerate the sand; the cleaned sand is subsequently reinjected upstream of the flocculator, at the front of the plant. The residue generated by this sand-ballast cleaning operation represents the excess sludge.
It will be noted that the existing ballast-type apparatus described in the literature, and especially in FR-P-1,411,792 and in FR-A-2,627,704, include a ballast-recycling step for obvious running-cost reasons. Moreover, in all the documents describing this technology it is specified that the ballast is always "cleaned", i.e. regenerated. This is because, the ballast, "coated" with the polymer, must have the maximum area of adhesion for the precipitation flocks produced chemically during coagulation. An effective physical cleaning is therefore indispensable for maximizing the binding area available.
The ballast is often sand, generally having a diameter of between 50 .mu.m and 150 .mu.m, usually called microsand.
The publication Journal Water SRT-AQUA, Vol. 41, No. 1, pp. 18-27, 1992 describes a curve relating the turbidity of the water produced to the diameter of the ballast particles, which demonstrates that this process becomes effective when the sand particles do not exceed 150 .mu.m, the results being even better with values of the order of 50 to 100 .mu.m.
It should be pointed out that the advantage of this technique of flocculation using a contacting mass consisting of a fine ballast essentially resides in the settling velocity, which may be from 20% to 200% greater than the velocities obtained by the flocculation processes using a contacting mass consisting of recirculated presettled sludge. Thus, when clarifying river water, the announced velocities through the lamellar modules are between 25 and 50 m.sup.3 /m.sup.2.h while the equipment implementing the flocculation process using sludge as the contacting mass is limited to velocities of between approximately 15 and 30 m.sup.3 /m.sup.2.h.
The essential drawbacks of this technique mainly stem from the fact that the ballast must provide two different functions:
These limitations or drawbacks can be imputed to the following characteristics:
in the case of "flocculation with sludge", the concentration in the reactor is approximately 1 g/l and the volume occupied by the sludge after five minutes of settling is approximately 10% of the initial volume; PA2 in the case of "flocculation with ballast (for example sand)", the ballast concentration in the reactor should reach at least 5 g/l, while the volume occupied by sludge after five minutes of settling is only approximately 1% of the initial volume; PA2 increasing the amount of ballast, desirable for obtaining a high contacting mass (and not for obtaining a high settling velocity), leads to an increase in the volume of sludge recirculated to the extracted-sludge treatment system, which treatment consists in separating the sludge from the sand so as to regenerate the latter. This operation is generally carried out by hydrocyclones supplied at high pressures, which operation therefore becomes very expensive from an energy consumption standpoint. In fact, and so as to limit the running costs, the volume of recirculated sludge is intentionally limited to between 5 and 10% of the volume treated and the ballast concentration in the reactor does not exceed 5 to 10 g/l: quite obviously this choice is incompatible with the possibility of optimizing the flocculation.
Various techniques are aimed at compensating for the deficit in the contacting mass resulting from the operating conditions described above, such as:
the use of additional flocculation energy (figures ranging up to 100 times the conventional flocculation energy may be mentioned) or
the use of even finer ballast particles, increasing the specific surface area (for example, particles having a diameter of between 10 and 50 .mu.m), is not conceivable, on the one hand, for energy cost reasons and, on the other hand, for reasons of difficulty in settling and in sand-flock separation.
In summary, the performance characteristics of flocculation with ballast are limited by three factors: